Recent advancements in communication technologies have enabled the widespread distribution of data over communication mediums such as the Internet and broadband cable systems. This increased capability has lead to increased demand for the distribution of a diverse range of content over these communication mediums. Whereas early uses of the Internet were often limited to the distribution of raw data, more recent advances include the distribution of HTML-based graphics and audio files.
More recent efforts have been made to distribute video media over these communication mediums. However, because of the large amount of data needed to represent a video presentation, the data is typically compressed prior to distribution. Data compression is a well-known means for conserving transmission resources when transmitting large amounts of data or conserving storage resources when storing large amounts of data. In short, data compression involves minimizing or reducing the size of a data signal (e.g., a data file) in order to yield a more compact digital representation of that data signal. Because digital representations of audio and video data signals tend to be very large, data compression is virtually a necessary step in the process of widespread distribution of digital representations of audio and video signals.
Fortunately, video signals are typically well suited for standard data compression techniques. Most video signals include significant data redundancy. Within a single video frame (image), there typically exists significant correlation among adjacent portions of the frame, referred to as spatial correlation. Similarly, adjacent video frames tend to include significant correlation between corresponding image portions, referred to as temporal correlation. Moreover, there is typically a considerable amount of data in an uncompressed video signal that is irrelevant. That is, the presence or absence of that data will not perceivably affect the quality of the output video signal. Because video signals often include large amounts of such redundant and irrelevant data, video signals are typically compressed prior to transmission and then expanded (i.e., decompressed) again after transmission.
Generally, the distribution of a video signal includes a transmission unit and a receiving unit. The transmission unit will receive a video signal as input and will compress the video signal and transmit the signal to the receiving unit. Compression of a video signal is usually performed by an encoder. The encoder typically reduces the data rate of the input video signal to a level that is predetermined by the capacity of the transmission or storage medium. For example, for a typical video file transfer, the required data rate can be reduced from about 30 Megabits per second to about 384 kilobits per second. The compression ratio is defined as the ratio between the size of the input video signal and the size of the compressed video signal. If the transmission medium is capable of a high transmission rate, then a lower compression ratio can be used. On the other hand, if the transmission medium is capable of a relatively low transmission rate, then a higher compression ratio can be used.
After the receiving unit receives the compressed video signal, the signal must be decompressed before it can be adequately displayed. The decompression process is performed by a decoder. In some applications, the decoder is used to decompress the compressed video signal so that it is identical to the original input video signal. This is referred to as lossless compression, because no data is lost in the compression and expansion processes. The majority of encoding and decoding applications, however, use lossy compression, wherein some predefined amount of the original data is irretrievably lost in the compression and expansion process. In order to expand the video stream to its original (pre-encoding) data size, the lost data must be replaced by new data. Unfortunately, lossy compression of video signals will almost always result in the degradation of the output video signal when displayed after decoding, because the new data is usually not identical to the lost original data. Video signal degradation typically manifests itself as a perceivable flaw in a displayed video image. These flaws are typically referred to as noise. Well-known kinds of video noise include blockiness, mosquito noise, salt-and-pepper noise, and fuzzy edges. The data rate (or bit rate) often determines the quality of the decoded video stream. A video stream that was encoded with a high bit rate is generally a higher quality video stream than one encoded at a lower bit rate.
Therefore, there is a need in the art for image enhancement that offsets some of the well-known image flaws caused by the conventional encoding and decoding processes. The image enhancement should be adaptive to provide varying degrees of enhancement, based on various constraints, such as transmission bit rate and image brightness.